The present application is directed to controls for lighting systems. It finds particular application in the efficient use of power in dimming applications, and will be described with particular reference thereto. It is to be appreciated, however, that the present exemplary embodiments are also amenable to other like applications.
Lighting control systems are frequently used to provide illumination to industrial buildings, commercial structures and other large spaces. Generally, lighting control systems include dimmable ballast systems to provide varying levels of light within the system based on any number of conditions such as an event, a time, etc. For multi-lamp fixtures, conventional dimming ballast techniques include discrete dimming (so-called “step-dimming”) and continuous dimming. One example of discrete dimming is a multiple-lamp discrete ballast in which one or more lamps are shut off to provide a lower light output. This is sometimes implemented using external controls to turn off individual ballasts or fixtures until the selected light level is achieved. Discrete dimming approaches, however, only provide a finite number of predefined lighting levels and transitions between these discrete levels are often perceptible by users.
Some continuous dimming designs operate multiple lamps in series with the power applied to the lamps being reduced for dimming. Series-connected dimming ballasts, however, suffer from inability to produce light when one or more lamps fail. Other proposed approaches include varying a DC bus amplitude via pulse width modulation (PWM) control to power a voltage or current fed inverter for driving one or more lamps, but this dimming control technique adds cost and may not provide the desired amount of dimming for certain applications. Continuous dimming techniques require auxiliary cathode heating power to keep the cathode operating within its normal temperature range at deep dimming levels. Failure to provide adequate auxiliary heat to the cathode at low current levels will result in cathode failure and short lamp life. Auxiliary cathode heating, however, contributes to inefficiency at dimming levels below a critical arc power level since the cathode heating power contributes no lamp light output. Thus, it may take as much as 75% of full lamp input power to provide 50% of full lamp lumens output.
Conventional lighting systems can also require all lights within an array to be dimmed to achieve a desired light output. Conventional dimming ballasts with continuous light level control are typically more expensive due to the necessary auxiliary cathode heating circuitry. Continuous dimming ballasts, moreover, suffer from reduced power efficiency. Thus, there is a need for improved fluorescent lamp dimming apparatus and techniques for efficiently providing varying lighting levels to match a desired lighting level while maintaining high efficiency and without lamp stress or damage or increased cost, thereby allowing a user to selectively achieve energy savings by dimming lighting installations.